Printed circuit boards (PCB) are used extensively throughout the electronics industry. Defects in a PCB are the result not only of problems in fabrication, but also from errors in features found in the artwork used therein. Over time, printed circuit boards have become very complex, with an increasing number of layers each having ever higher densities of electrical components. The defects which occur during production are therefore correspondingly more difficult to identify by manual inspection. Manual inspection by workers is a very costly process with uneven results. Each board must be handled individually by a worker who must locate and analyze each defect.
As a result, a series of computer based systems have been developed to optically inspect each printed circuit board for defects. PCB artwork sometimes is inspected by these systems as well. Typically, the optical inspection occurs at the end of the fabrication of each PCB layer. Known optical inspection systems, such as that disclosed in U.S. Pat. No. 4,578,810 optically scan a printed circuit board and compare the image thereof against a reference image stored in computer memory. Should a defect be noted, the known systems will record the approximate size and location thereof on the printed circuit board in a data file. Certain ones of these systems will position a camera or other optical inspection device over the defect to allow for examination by a worker.
However, these known systems are not able to accurately categorize the defects found on a printed circuit board to automatically determine whether the board must be rejected offhand, or whether the defects are of a type, magnitude and location that they may be repaired. For example, excess material can be removed from a metalized printed circuit board, but missing material which translates into electrical opens on a printed circuit board are rarely repaired. Consequently, it is still necessary that a worker visually inspect each board to determine the full extent and type of the damage which has been previously identified by the automatic defect inspection system.
Computer based systems of this type are referred to as verification stations or inspection stations and are marketed by the AOI Systems, Inc. company of Balowa, Mass., the Multi-Line Technology company of Farmingdale, N.Y. and the Operations Technology, Inc. of Blairstown, N.J. These known verification stations are characterized by a controller which receives a data file containing the digitized defect information obtained during an earlier scan of that printed circuit board by a defect detection system as detailed above. The controller generates signals for a mechanism that moves the printed circuit board relative to an optical microscope or camera so that the microscope is positioned over a defect, allowing for direct visual inspection or viewing via a television monitor.
The worker views each defect identified by the defect detection systems in sequence and determines whether the PCB can or cannot be repaired. A PCB that cannot be repaired is rejected outright. The printed circuit boards which can be repaired must also be marked in some manner as to the location of the defect(s). Moreover, certain defects are of such a small magnitude that no repair work is necessary. This determination can only be made by the worker after visual inspection.
In a high volume production environment, the costs associated with verification can assume a substantial portion of the total cost of the board. Every step in the verification process which is simplified or eliminated without a corresponding reduction in accuracy or speed results in a substantial savings in manufacturing cost. As the verification procedure must take place for each printed circuit board which is found to have at least one defect, the design of the verification station has a dramatic effect on the final cost of the printed circuit board.
Known verification stations are slow and are not adapted to be integrated into a fabrication line. The time required for a worker to manually handle and position each PCB and verify the defects thereon is substantial. Moreover, these verification stations require a separate facility for accomplishing repairs to a board or allow for a repair to be performed only awkwardly.
It would be advantageous to have a verification station which is low in initial cost and would be especially adapted to allow speedy identification of defects on a printed circuit board and allow for an inexpensive and accurate repair thereof. The present verification system is drawn towards such an invention.